It doesn't. It swaps gear around between your available gear, looking for upgrades. When it does find an upgrade, it's quite possible that it has swapped rings/trinkets in the process. In a future version, I'll try to detect those swaps in the results,
and ignore them. In the mean time, it's finding the upgrade in whatever *else* it's changing.

Just to be sure there is no confusion though, I'm not saying the ring moves from one finger to the other because Rawr is bringing in a different ring to one finger, I'm noticing that Rawr is just swapping the two existing rings (or trinkets) with each other
for no net difference.

Or it really just swapped the rings and gives the same result, it does that sometimes. The reason is that while it is optimizing it does not look at your character and just creates a new character from scratch based on available items. As a result it usually
ends up with rings and trinkets in a different slot.